Low-pressure axial direction excitation type f2 laser oscillator

ABSTRACT

An axial direction excitation type F 2  laser apparatus ( 11 ) comprising a discharge tube ( 1 ) consisting of an insulating cylinder and metal electrodes ( 2, 3 ) at both ends of thereof, and a reflecting mirror or a transmitting mirror constituting a resonator outside the electrodes. A high voltage for pulse discharge is applied to the discharge electrodes ( 2, 3 ) from a drive circuit. The total gas pressure in the discharge tube ( 1 ) is set in a range between 10 Torr. and 100 Torr., the concentration of F 2  gas to the total gas is set to be in a range between 0.2% and 2.0%. The low-pressure axial direction excitation type F 2  laser apparatus having small size high efficiency can be provided at a low cost.

TECHNICAL FIELD

The present invention relates to a low-pressure axial direction excitation type F₂ laser apparatus. In particular, the present invention relates to a low-pressure axial direction excitation type F₂ laser apparatus that is useful for a light source of a semiconductor exposure apparatus or the like, that oscillates at a low pressure, that is small in size, high in efficiency and low in cost.

BACKGROUND ART

Conventionally, as the semiconductor process becomes finer, the exposure wavelength of the laser used in the semiconductor exposure apparatus is made short. Excimer lasers such as a krypton fluoride (KrF: 248 nm) laser and an argon fluoride (ArF: 193 nm) laser have been put to practical use. In addition, the F₂ laser (F₂: 157 nm) oscillating in the vacuum ultraviolet ray region is anticipated as a light source of the next generation. Furthermore, in the F₂ laser, the photon energy is as large as approximately 7.9 eV. Even in a material having a large band gap, such as silica, the absorption coefficient is large. Therefore, new application to materials that have been said to be difficult in working is also anticipated.

Furthermore, gas used in the F₂ laser is simple mixed gas composed of He and F₂. The gas does not use expensive gas, such as Ne or Xe, and it does not contain a component, such as Ar, that tends to localize the discharge. Therefore, the gas used in the F₂ laser has a merit that stable discharge can be easily obtained.

In addition, in the study of the F₂ laser, oscillation up to approximately 260 mJ/pulse is already obtained and its aspect suited to a higher output has been exhibited. If the F₂ laser is put to practical use, it is anticipated to become a laser that can be applied to various fields.

As described above, the F₂ laser is a short-wavelength laser having a great advantage. However, in the existing circumstances in which the traditional F₂ laser of the transverse direction excitation type, in which the direction of laser light becomes perpendicular to the discharge current direction, and of ultraviolet ray preliminary ionization type, there are a large number of problems. It has been considered that it is difficult to overcome those problems with the traditional way of thinking.

First, a high gas pressure is required for the F₂ laser. At least 3 atm is required, and in the case where the required gas pressure is high, a pressure as high as 10 atm is required. Therefore, it has been considered that it is necessary to use a large-sized chassis having a high strength. Therefore, the apparatus as a whole becomes large and expensive.

Secondly, for conducting high repetition operations, it is necessary to let gas flow at a high speed. For example, for causing operation at a repetition frequency exceeding 5 kHz, it has been considered that it is necessary to let flow gas at a speed exceeding several tens n/sec. This results in a problem of power required to drive a fan exceeding 20 kW.

In addition, there is a phenomenon called gas life. If the laser is activated, its output falls within a relatively early time. Unless the laser gas is exchanged frequently, the output cannot be maintained. As regards the gas life, it has been coped with by exchanging the gas, while how the cause material decreases the laser output is unknown.

Since in the F₂ laser the self-excitation voltage of the discharge is high and the discharge is apt to become unstable. In the transverse direction excitation type laser, therefore, the electrode interval is set to approximately 10 mm. However, understanding of homogeneous discharge generated in the narrow electrode gap of approximately 10 mm is insufficient. In the time of approximately 10 ns during which discharge is formed, even electrons cannot cross the gap between the electrodes, and move little. Nevertheless, it is necessary to generate homogeneous discharge. The occurrence condition of the homogeneous discharge has been looked for experientially. That is details of the development that has been conducted heretofore.

In addition, in a lamp with F₂ gas sealed therein at a low pressure, a luminous efficiency exceeding 50% is obtained in continuous lighting that extends over several thousands hours. Therefore, it is expected that the cause of the low coefficient and the short gas life exists in the structure itself of the laser apparatus. Indeed, after the present inventors' study conducted for approximately 10 recent years, a conclusion that most of these are mere structural problems of the transverse direction excitation and the ultraviolet ray preliminary ionization type laser is reached.

For example, as for the high gas pressure required for the laser apparatus, it is considered that in a state in which the electrode gap in the transverse direction excitation type laser is narrow and the preliminary ionization is sufficiently conducted the discharge start voltage becomes a function of only the pressure and the effective operating voltage does not rise unless a high gas pressure is used. The reason why the gas must be let flow at high speed is also based on the narrow electrode gap in the transverse direction excitation type. If remaining electric charge exists, the discharge is localized. Therefore, it is considered that the gas is let flow to remove the remaining electric charge.

As for the gas life, it relates to the type in which the discharge gas is subject to preliminary ionization using ultraviolet rays. Since impurities stored in the discharge gas absorbs the preliminary ionization light, the quantity of light arriving at the discharge space decreases, resulting in a state of insufficient preliminary ionization. It is considered that as a result the homogeneity in the discharge falls and the output decreases.

Therefore, the present inventors have noticed a supposition that those problems might be solved by fundamentally altering the structure of the laser apparatus, and decided to adopt an axial direction excitation type laser, in which the optical axis of laser light and the discharge current pass through the same path, instead of the conventional transverse direction excitation and ultraviolet ray preliminary ionization type laser. In this axial direction excitation type laser, the excitation discharge occurs on the optical axis, and consequently the electrode gap becomes as wide as a range of 15 cm to 30 cm and it becomes possible to obtain a high discharge start voltage even at a low gas pressure. Therefore, the gas pressure can be made low, and consequently the structure of the laser apparatus becomes very simple. As a result, not only remarkable size reduction of the apparatus becomes possible, but also a blower for preventing the remaining charge in the discharge space from affecting the homogeneity of the discharge becomes unnecessary.

As regards the increase of impurities, which affect the gas life, it also becomes possible to seal and use a discharge tube over a long period of time by hard-sealing components. In addition, in the axial direction excitation type laser, preliminary ionization can be conducted from the outside of the discharge tube without using ultraviolet light. Therefore, there is a possibility that the gas life, which has posed a problem in the conventional transverse direction excitation type laser, will matter little. Indeed, for example, in a copper vapor laser, it has become clear that an output of kW class with repetition of several hundreds kHz is obtained with a low pressure gas without letting flow the gas, owing to the use of the axial direction excitation type laser.

Therefore, the present inventors have adopted the axial direction excitation type laser as the F₂ laser as well, and developed an axial direction excitation type F₂ laser apparatus having a stable output and a high efficiency and allowing high repetition (Japanese Patent Application Publication No. 2000-265435).

In the developed axial direction excitation type F₂ laser, however, the laser oscillates at a pressure near 1 atm. Therefore, the present inventors have continued studies in order to implement a highly efficient axial direction excitation type F₂ laser that generates oscillation at a further low pressure.

The present invention has been achieved in view of the circumstances heretofore described. The present inventors aim at providing a small-sized, highly efficient, low-cost, low-pressure axial direction excitation type F₂ laser apparatus oscillating at a pressure that is as low as a range of {fraction (1/20)} to {fraction (1/100)} of the conventional laser apparatus.

DISCLOSURE OF THE INVENTION

As a solution to the above-described aim, the present invention firstly provides a low-pressure an axial direction excitation type F₂ laser apparatus comprising: a discharge tube consisting an insulating cylinder and metal electrodes at both ends thereof; and a reflecting mirror or a transmitting mirror constituting a resonator outside the electrodes, a high voltage for pulse discharge being applied to the discharge electrode from a drive circuit, an optical axis of laser light generated by oscillation and a main discharge current passing through same path, wherein a total gas pressure in the discharge tube is set in a range between 10 Torr. and 100 Torr., and the concentration of an F₂ gas to the total gas is set to be in a range between 0.2% and 2.0%.

The present invention secondly provides the low-pressure axial direction excitation type F₂ laser apparatus, wherein the total gas pressure in the discharge tube is set in a range between 35 Torr. and 45 Torr.

The present invention thirdly provides the low-pressure axial direction excitation type F₂ laser apparatus, wherein in order to avoid laser light absorption due to a long life of a lower level concerning laser oscillation, an excitation density is controlled and gain sustaining time is controlled by changing a discharge voltage or capacitance.

The present invention fourthly provides the low-pressure axial direction excitation type F₂ laser apparatus, wherein a gain length is shortened and thereby a length of the resonator is made as short as 10 cm to 30 cm to shorten round-trip time of light and make the resonator sufficiently effective.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram exemplifying a low-pressure axial direction excitation type F₂ laser apparatus and a drive circuit according to the present invention;

FIG. 2 is a diagram exemplifying a configuration of a gain measurement system in a low-pressure axial direction excitation type F₂ laser apparatus according to the present invention;

FIG. 3 is a graph showing a gain time waveform and a voltage and current waveform of a low-pressure axial direction excitation type F₂ laser apparatus according to the present invention;

FIG. 4 is graphs showing dependence of a gain coefficient upon F₂ partial pressure and total pressure in a low-pressure axial direction excitation type F₂ laser apparatus according to the present invention;

FIG. 5 is a graph showing dependence of a laser output upon a gas pressure in a low-pressure axial direction excitation type F₂ laser apparatus according to the present invention; and

FIG. 6 is a graph showing a delay time of laser oscillation in a low-pressure axial direction excitation type F₂ laser apparatus according to the present invention.

Reference numerals in the drawings denote the following components:

-   -   1 discharge tube     -   2, 3 cylindrical electrode     -   4 spark gap     -   5 reflecting mirror     -   6 transmitting mirror     -   7 capacitor of main discharge circuit     -   8 peaking capacitor     -   9 capacitor of preliminary ionization circuit     -   10 copper foil     -   11 low-pressure axial direction excitation type F₂ laser         apparatus     -   12 transverse direction excitation type F₂ laser discharge tube     -   13 coaxial discharge tube (amplifier tube)

BEST MODE FOR CARRYING OUT THE INVENTION

A low-pressure axial direction excitation type F₂ laser apparatus according to the present invention is an axial direction excitation type F₂ laser apparatus comprising a discharge tube consisting an insulating cylinder and metal electrodes at both ends thereof, and a reflecting mirror or a transmitting mirror constituting a resonator outside the electrodes, a high voltage for pulse discharge being applied to the discharge electrode from a drive circuit, an optical axis of laser light generated by oscillation and a main discharge current passing through same path. Highly efficient laser oscillation can be caused at a low pressure by setting a total gas pressure in the discharge tube equal to a value in a range between 10 Torr. and 100 Torr., and setting the concentration of an F₂ gas to the total gas equal to a value in a range between 0.2% and 2.0%. Especially, by setting the total gas pressure in the discharge tube to a value in a range between 35 Torr. and 45 Torr., a further high output can be obtained, and highly efficient laser oscillation becomes possible.

Typically, in the case of the F₂ laser, it is filled up with mixed gas composed of F₂ and inert gases such as He, and excitation discharge is caused in the gas. In this excitation discharge, electrons generated by the ionization are accelerated by the applied electric field, and the electrons collide with F₂ in the discharge tube and raise the F₂ to the excitation state. As a result, the laser gas generates an optical gain at the laser wavelength, and laser oscillation is caused by repetitive reflection at a resonator.

The F₂ laser thus oscillates. According to a mechanism considered heretofore, He (or inert gas such as Ne) is excited, and He and F₂ collide with each other. By receiving energy, F₂ is dissociated to emit light. In oscillation of the F₂ laser at a low pressure according to the present invention, however, rise of the gain begins immediately after the start of the main discharge. Therefore, it is considered that the oscillation is caused by excitation of the F₂ directly to an upper level due to the collision of electrons during the discharge. It is considered that oscillation at a low pressure is caused by such a mechanism.

Since oscillation at a low pressure becomes possible as described above, the voltage withstand property in the laser apparatus also becomes unnecessary and remarkable size reduction of the laser apparatus becomes possible. Furthermore, since high-pressure gas is not used, F₂ gas having toxicity can be used at a pressure lower than the atmospheric pressure. Therefore, it is not necessary to provide the apparatus with high pressure gas piping, and the risk that poison gas will leak becomes nil.

In the axial direction excitation type F₂ laser apparatus as described above, in order to avoid laser light absorption due to a long life of a lower level concerning laser oscillation, an excitation density is controlled and gain sustaining time is controlled by changing a discharge voltage or capacitance. As a result, highly efficient laser oscillation becomes possible.

In an axial direction excitation type F₂ laser apparatus, a gain length is shortened and thereby a length of the resonator is made as short as 10 cm to 30 cm to shorten round-trip time of light and make the resonator sufficiently effective. As a result, oscillation having a narrow wavelength and a favorable directivity can be obtained. In the case of F₂, the life of the upper level is considered to be approximately 3.5 ns. Therefore, the round-trip time of light needs to be less than approximately 3.5 ns. Since light travels at a speed of 3.3 ns per meter, a transit distance less than 1 m and a round-trip distance of 0.5 m or less become necessary conditions. By making the resonator length as short as 10 to 30 cm as compared with the conventional resonator length of 0.5 to 1 m, the round-trip time of light can be shortened and the resonator can be made sufficiently effective.

Hereafter, embodiments will be shown with reference to the accompanying drawings, and the mode for carrying out the present invention will be described in further detail. Of course, the present invention is not limited to the following examples, and it is a matter of course that various modes are possible as regards the details.

Embodiments

<Embodiment 1>

FIG. 1 shows a schematic diagram of a discharge tube in a low-pressure axial direction excitation type F₂ laser apparatus according to the present invention, and a drive circuit. As shown in FIG. 1, a discharge tube 1 is a ceramics pipe having an inside diameter of 5 mm and a length of 15 cm. Cylindrical electrodes 2 and 3 serving also as mirror holders are attached to respective ends of the discharge tube 1. These cylindrical electrodes 2 and 3 have a gas inlet and a gas outlet, respectively. A high voltage pulse generated from a capacitance shift type discharge circuit using a high pressure type spark gap 4 is applied to the electrodes 2 and 3 disposed at ends. A reflecting mirror 5 and a transmitting mirror 6, which form a resonator, are attached outside of these electrodes 2 and 3, respectively.

In this circuit, a capacitor for conducting preliminary ionizing on the discharge tube 1 is also connected in parallel to the main discharge circuit. As for capacitance in a power supply circuit, a capacitor 7 in the main discharge circuit has capacitance of 5.4 nF, a peaking capacitor 8 has capacitance of 560 pF, and a capacitor 9 in a preliminary ionization circuit has capacitance of 50 pF. As for the preliminary ionization, a high voltage is applied to copper foil 10 wound around an outer periphery of the discharge tube 1, and consequently weak corona discharge is generated on a wall surface in the low-pressure discharge tube 1. The preliminary ionization is thus conducted automatically. By sealing He and F₂ gas diluted in He of approximately 5% in the discharge tube 1, laser oscillation is obtained.

First, gain measurement was conducted in order to look for an optimum condition of a low-pressure axial direction excitation type F₂ laser apparatus 11 in this example.

Since the gain measurement of a pulse laser in a vacuum ultraviolet region, such as F₂ laser, involves difficulty, first the discharge tube was activated as a N₂ laser, in which handling of gas is simple and laser light can be propagated in the atmosphere, and a gain measurement system and a measurement technique was completed.

A configuration of its gain adjustment apparatus is shown in FIG. 2. A transverse direction excitation type F₂ laser discharge tube 12 having a length of 30 cm was used as the oscillator. First, N₂ gas was put in the discharge tube 12 and oscillation was caused. The oscillation light was passed through a coaxial discharge tube (amplifier) 13, and its output was measured. Each laser apparatus has a gap switch, and it is typically difficult to achieve the synchronization. Therefore, a technique of measuring mutual oscillation times by using jitter of both switches and thereby obtaining timing of incident light was adopted.

In this type, the pressure of one of the switches is changed and coarse time is determined. Since the time difference that fluctuates due to mutual jitter can be used as spread, accurate dependence of the gain upon time with simple operation can be obtained.

After this gain measurement method was established in the N₂ laser, the gain in the F₂ laser was measured. By the way, in order to prevent laser light from being absorbed by oxygen in the atmosphere, the path through which laser light passes was replaced by nitrogen gas and measurement was conducted. Here, the discharge length in the discharge tube of the F₂ laser is 15 cm, and the used voltage is 20 kV.

A temporal waveform of the gain in the F₂ laser measured by using the method described above, an actually measured voltage waveform, and a current waveform obtained by a circuit calculation are shown in FIG. 3. In FIG. 3, V represents a voltage waveform, I represents a current waveform, G represents a temporal waveform of the gain, “a” represents an output of only the amplifier, and “b” represents a total output of the oscillator and the amplifier. FIG. 4(a) and (b) show dependence of a peak gain coefficient upon F₂ partial pressure and total pressure.

As a result of this measurement, it was found that the gain rise lagged little behind the start of the main discharge unlike in the case of high pressure. Furthermore, in a time zone lagging behind the discharge start in the amplifier by approximately 60 ns, the output transmitted through the amplifier assumes a value close to the fluorescent amplified (ASE light) output of the amplifier alone, and laser light from the oscillator is not transmitted through the amplifier. This phenomenon is seen in the case of N₂ laser as well. It can be said that the distribution of the lower level of the laser becomes large, and absorption of laser light occurs.

Heretofore, a large number of gain measurements in the F₂ laser have been performed. However, a result of measurement of such strong absorption has not been seen. As for the reason, it is considered that much attention has not been paid to the latter half of the pulse in the conventional measurement, or collision de-ionization of the lower level occurs at a high pressure that amounts to several atm. Even in the gain measurement at 1 atm, however, similar strong absorption is seen. Therefore, the possibility of the former case is high.

In the case where the discharge length of the discharge tube was 15 cm and the voltage in use was 20 kV as a result of the measurement, the maximum gain coefficient was 13.5%/cm, and the concentration of F₂ gas (partial pressure of F₂) and the total pressure with respect to the total gas were 1.5% (0.6 Torr.) and 40 Torr., respectively, as shown in FIG. 4. This gain coefficient has a value higher than a gain coefficient 11%/cm obtained when mixed gas (0.25%) having a high pressure (1 atm) and having a smaller concentration of F₂ gas is put in the discharge tube. In view of the behavior in a wide range, the gain decreases as the pressure is decreased from the state of the high atmospheric pressure, as shown in FIG. 4. However, the gain rises again in the vicinity of 40 Torr. At this time, the partial pressure of F₂ is 0.6 Torr. Therefore, an optimum concentration of F₂ is approximately 1.5%. The temporal rise of the gain also lags by approximately 15 ns in the case of a high pressure. On the other hand, at a low pressure, a delay time is not seen and the duration time of the gain also becomes long.

<Embodiment 2>

In the gain measurement in the low-pressure axial direction excitation type F₂ laser apparatus according to the present invention, a high gain was obtained in the low-pressure gas, as described above. Therefore, a shift to the oscillation experiment was conducted. As for the discharge length in the discharge tube and the voltage in use, the discharge length is 15 cm and the voltage in use is 20 kV in the same way as the embodiment 1. At the beginning, oscillation was attempted with the incidence side window of the amplifier used in the amplification experiment being replaced by a total reflection mirror. Since an aluminum evaporation mirror was used, however, air leaked from protection coating of MgF₂ provided at the surface although the quantity of the leak was slight, and oscillation was not conducted successfully. With the mirror placed outside the transmitting window and that portion also subjected to nitrogen purge, therefore, oscillation experiment was conducted.

Since one extra window is provided and a window material of CaF₂ is used as the exit window, therefore, it is expected that the resonator remarkably gets out of the optimum condition.

In fact, oscillation experiment was conducted, and the output energy was measured with the gas mixture ratio and the total pressure of gas changed. Its result is shown in FIG. 5. In the case where the discharge length is 15 cm and the voltage is 20 kV, approximately 140 μJ is obtained as the highest value of the output with the F₂ concentration of 1.25%, the F₂ partial pressure of 0.5 Torr., and the total pressure of 40 Torr. This value exceeds 100 μJ for the high-pressure axial direction excitation type F₂ laser.

Temporal relations between the oscillation waveform and the voltage waveform are shown in FIG. 6. In FIG. 6, V represents a voltage waveform, L represents an oscillation waveform at the low pressure, whereas H represents an oscillation waveform at the high pressure. The voltage waveform shows one in the case of the low pressure. The oscillation waveform at the high pressure is shown so as to be aligned in discharge start time.

As shown in FIG. 6, the time of oscillation remarkably differs according to whether the pressure is high or low. In the low pressure, there is little delay time from the discharge start. On the other hand, at the high pressure, a delay of 15 ns is caused.

On the basis of the result described above, it is presumed that this oscillation at the low pressure is generated by excitation of F₂ directly to the upper level due to electron collision in the discharge. It is considered that this oscillation at the low pressure is remarkably different in the mechanism for exciting F₂ from the conventional case where the high gas pressure is used. On the basis of the fact that it has been confirmed that strong absorption is generated in the latter half of the pulse and the fact that the pulse width becomes short to 4 ns in the case of stronger excitation, it is necessary in the F₂ laser to design the laser apparatus while considering the distribution of the lower level, unlike in the case of other excimer lasers.

Furthermore, in the axial direction excitation type F₂ laser apparatus of the embodiment, it is easy to carry out hard seal because the structure is simple, and the discharge is hard to degrade even if the remaining electric charge exists. Conversely, the remaining electric charge can serve also as the preliminary ionization, and consequently the axial direction excitation type F₂ laser apparatus of the embodiment is suitable for high repetition operations. Furthermore, it has already been confirmed in the N₂ laser that the seal operation is possible, continuous oscillation of 100 Hz is possible without letting flow gas, and operation corresponding to 1 kHz is possible if the duration is extremely short. Therefore, it is considered that they can be applied to the F₂ laser as well as they are.

Furthermore, in the axial direction excitation type laser, ultraviolet light is not used for the preliminary ionization. Therefore, there is a possibility that the gas life which has posed a problem in the conventional transverse direction excitation type laser matters little. According to the development conducted from now on, it is considered that rapid performance improvement and a laser apparatus that can withstand the practical use in the severe industrial world are implemented.

In the experiment described above, it has been found that the F₂ laser that generates oscillation in the vacuum ultraviolet ray region has the same characteristics as those of the very simple N₂ laser. Therefore, it is considered that the seal technique, high repetition operations, and the switch less operation without high voltage switches, which have been implemented with the N₂ laser, can be applied easily. Since the F₂ gas itself has reactivity, however, it might be difficult to maintain the life of the discharge tube with complete sealing unlike the N₂ laser. However, the possibility that an apparatus that can continue the oscillation with only simple supply of F₂ is high.

INDUSTRIAL APPLICABILITY

As heretofore described in detail, the present invention provides a small-sized, highly efficient, and low-cost low-pressure axial direction excitation type F₂ laser apparatus that generates oscillation at a pressure that is as low as {fraction (1/20)} to {fraction (1/100)} of the pressure of the conventional apparatus. 

1. A low-pressure an axial direction excitation type F₂ laser apparatus comprising: a discharge tube consisting an insulating cylinder and metal electrodes at both ends thereof; and a reflecting mirror or a transmitting mirror constituting a resonator outside the electrodes, a high voltage for pulse discharge being applied to the discharge electrode from a drive circuit, an optical axis of laser light generated by oscillation and a main discharge current passing through same path, wherein a total gas pressure in the discharge tube is set in a range between 10 Torr. and 100 Torr., and the concentration of an F₂ gas to the total gas is set to be in a range between 0.2% and 2.0%.
 2. The low-pressure axial direction excitation type F₂ laser apparatus according to claim 1, wherein the total gas pressure in the discharge tube is set in a range between 35 Torr. and 45 Torr.
 3. The low-pressure axial direction excitation type F₂ laser apparatus according to claim 1, wherein in order to avoid laser light absorption due to a long life of a lower level concerning laser oscillation, an excitation density is controlled and gain sustaining time is controlled by changing a discharge voltage or capacitance.
 4. The low-pressure axial direction excitation type F₂ laser apparatus according to claim 1, wherein a gain length is shortened and thereby a length of the resonator is made as short as 10 cm to 30 cm to shorten round-trip time of light and make the resonator sufficiently effective.
 5. The low-pressure axial direction excitation type F₂ laser apparatus according to claim 2, wherein in order to avoid laser light absorption due to a long life of a lower level concerning laser oscillation, an excitation density is controlled and gain sustaining time is controlled by changing a discharge voltage or capacitance.
 6. The low-pressure axial direction excitation type F₂ laser apparatus according to claim 2, wherein a gain length is shortened and thereby a length of the resonator is made as short as 10 cm to 30 cm to shorten round-trip time of light and make the resonator sufficiently effective.
 7. The low-pressure axial direction excitation type F₂ laser apparatus according to claim 3, wherein a gain length is shortened and thereby a length of the resonator is made as short as 10 cm to 30 cm to shorten round-trip time of light and make the resonator sufficiently effective.
 8. The low-pressure axial direction excitation type F₂ laser apparatus according to claim 5, wherein a gain length is shortened and thereby a length of the resonator is made as short as 10 cm to 30 cm to shorten round-trip time of light and make the resonator sufficiently effective. 